Field of the Disclosure
The present invention relates to a USY catalyst, a method for modifying a USY zeolite, preparation of a hydrocracking catalyst, and its use for hydrocracking of petroleum fractions such as vacuum gas oil. The hydrocracking catalyst is employed for preparing valuable light and medium boiling range hydrocarbons from heavy hydrocarbons of vacuum gas oil fractions of crude oil.
Description of Related Art
In the development of hydrocracking processes, two basic types of catalyst have been used in hydrocracking: amorphous silica/alumina and crystalline silica/alumina (zeolites). Both types have distinct performance characteristics. Dual catalytic systems have also been developed to meet the demand of refining situations where higher middle distillate production and good quality products are demanded. These are two-stage processes: the first stage resembles hydrotreatment where the main reactions are hydrodesulfurization, hydrodenitrogenation and some conversion; and in the second stage, complete conversion by cracking is performed.
The basic principle in hydrocracking catalyst design is to obtain a balance between hydrogen transfer-functionality and cracking-functionality to achieve a desired slate of products without coke deposition. The choice of catalyst depends, apart from economic consideration, on factors such as feed properties and characteristics of desired products. For example, while choosing the appropriate hydrogenating/dehydrogenating function, it must be considered whether the feed contains poisons such as sulfur, nitrogen and oxygen which make the use of noble metals infeasible. Also, if a catalyst with a given acidity is to be used for obtaining products with a boiling point similar to the feed and with high saturation of the aromatic fraction, a noble metal, instead of a non-noble metal, is recommended as the hydrogenating function. On the other hand, catalysts with a high acidity/hydrogenation ratio should be prepared if products with a ratio of branched to normal paraffins higher than the feed, and therefore, gasoline with a higher octane number, is to be obtained. Excessive acidity tends to cause coking, but this can be overcome with higher hydrogen functionality. Higher hydrogen pressure is also effective but has other disadvantages. Thus, in order to prepare a suitable hydrocracking catalyst, a good balance between the two functions must be obtained.
Modifying the acidity of the zeolite and controlling the hydrogenation function by incorporating active metals are means to change catalyst function and activity. The acidity modifications include dealumination by steam treatment, acid leaching, phosphorus impregnation or the combination of these methods. Often, hydrocracking is carried out in the presence of a catalyst containing sulfided Ni-W or Ni-Mo metals. However, these catalysts have lesser hydrogenation functionality and produce more aromatics than saturates, which are required as high quality products. A high performance hydrocracking catalyst that gives higher conversion and more saturates under hydrocracking conditions is desirable but has not before been prepared.